The present invention relates to an aspherical structure, and particularly to a method for producing an aspherical lens in the optical field.
In the field of optical communication, improvement in speed and grade of optical signal processing has been demanded with the advance of increase in communication capacity. Optical elements need to be arrayed for parallel processing of optical signals so that optical coupling between the optical elements can be made with higher efficiency. A lens array is an optical element which fulfills an important role in such optical coupling.
For example, after divergent light rays emitted from a light sources an optical fiber end surface, or the like, are converted into parallel light rays and subjected to signal processing by an optically functional device, a pair of collimator lenses are used for converging and coupling the light rays to a photo detector, an optical fiber end surface, or the like. The distance between the collimator lenses maybe required to be increased in accordance with the optically functional device inserted in between the collimator lenses. In this case, it is necessary to enlarge the aperture of each lens. When each lens is a spherical lens made of a homogeneous material, it is however impossible to make the inter-lens distance larger than a certain value because aberration is inevitably produced with respect to light rays divergent from the optical axis.
An aspherical lens is used for solving this problem. When the surface shape of the lens is designed appropriately, aberration in the peripheral portion of the lens can be corrected. Such an aspherical lens is generally mass-produced by press molding using a molding tool formed by precision machining. Therefore, a molding tool producing technique is very important for production of the aspherical lens.
If a lens array molding tool is produced by machining, however, a cutter abrades. Hence, in the case of a lens array constituted by a large number of lens elements, there is a problem that the size of the molding tool varies in accordance with the respective lens elements. This problem can be solved when the molding tool is produced by etching. It is however difficult to form a molding tool for an aspherical structure because nothing but a spherical surface can be processed as long as the material of the molding tool is a homogeneous isotropic material.
The invention is developed to solve the problem and an object of the invention is to provide a method for producing an aspherical structure such as an aspherical lens array in which uniform lens elements can be formed.